Kingdom: Animalia, Phylum: Hemichordata
The Hemichordata is a phylum of worm shaped marine animals that are often thought of as a group closely related to the Echinodermata. They were present in the Lower or Mid-Cambrian period so that there many fossils called graptolites.
Their bodies are divided into three parts: (1) a proboscis, (2) collar and (3) trunk. They possess and open circulatory system and a complete digestive tract. The gut does not have well developed muscles and food is transported primarily through it by cilia that cover the inside surface.
A stomochord or diverticulum of the foregut occurs that is probably related to the notochord of the Chordata. However, this may be the result of convergent evolution rather than homology. A hollow neural tube is found in some species, which is thought to be a primitive trait that they share with the common ancestor of the Chordata.
General Characteristics of Hemichordata
Body Regions. -- There are three: a proboscis that resembles an acorn, a collar and a trunk.
Gill Slits. -- These occur along the sides of the trunk. They are closely allied with the Chordata.
Body Plan. -- The mouth is located between the proboscis and collar. The pharynx has a diverticulum. The gill slits open through the body wall and to the outside of the animal. A diverticulum runs from the pharynx, which has been homologized with a notochord.
Nerve Cords. -- There is a dorsal nerve cord, which may be hollow in a few forms. There is also a ventral nerve cord.
Embryology. -- The egg undergoes cleavage similar to the starfish and Amphioxis. They have a Tornaria larva, which resembles the Auricularia larva of the Holothuroidea (Echinodermata). This suggests an echinoderm and chordate relationship.
Economic Importance. -- There is no economic importance, however the animals give off an iodoform odor.